Recent research has shown that usage of the radio spectrum is often fairly inefficient. One key factor in this is the current spectrum licensing regime. That is, some part of the radio spectrum is licensed to a party, such as an operator of a radio communications system, who is given an exclusive right to use this part of the radio spectrum. For example, even though significant parts of the useful spectrum is licensed, several measurements (see, e.g. T. Erpek, K. Steadman, D. Jones, “Spectrum Occupancy Measurements: Dublin, Ireland, Collected On Apr. 16-18, 2007”, Shared Spectrum Company Report, 2007) indicate that some parts of this spectrum are highly underutilized. Therefore, a more flexible use of the radio spectrum has become a research intensive subject within which the aim is at optimizing, i.e. maximizing, the usage of the available radio spectrum.
To address the above-indicated issues, the FCC (Federal Communications Commission) took, in 2005, an initiative to open up for so-called secondary use of the spectrum. That is, radio resources that, e.g. under a license, are owned by one party—the primary party—may also be used by others (secondary users) for purposes (secondary use) that do not fall within the operation (primary use) of the primary party. Consequently, a secondary user may use radio resources owned by the primary party without having a license and without having made an explicit agreement with the primary party. A requirement for accessing radio resources as a secondary user is that the primary party should not be exposed to harmful interference caused by the secondary use. Therefore a secondary user is allowed to transmit over radio resources owned by the primary party if it can be ensured that the primary party is, for all practical purposes, not affected.
In order to determine when a secondary user can transmit without negatively affecting the primary user some kind of mechanism must be provided. Thus, secondary users should be allowed and the mechanism should enable secondary users to access primary users' spectrum when no primary user's communication quality are (substantially) affected. One approach for assessing the (instantaneous) primary usage of spectrum is to perform sensing, i.e., using sensors (radio receivers) with the goal of measuring the presence of primary transmissions. If a sensing result does not indicate primary usage the measured spectrum may be utilized for secondary access. This type of mechanism is often denoted sensing-based opportunistic spectrum access. The overall objective, simply stated, is to increase capacity as well as individual user throughput. Existing approaches are discussed and evaluated in www.academypublisher.com/jcm/vol02/no02/jcm02027182.pdf, and www.eecs.berkeley.edu/.about.sahai/Papers/ICC06_final.pdf.
Briefly, it can be said that collaborative sensing is built on the idea that collecting multiple sensors' information leads to increased probability of detection and reduced false-alarm probability.
Also a distributed algorithm for collaborative adaptive sensing of fields in an underwater environment is presented in: //perso.eleves.bretagne.ens-cachan.fr/.abouthuguenin/UUST07_Distrib-utedSampling.pdf.
The idea is in part to, in a distributed manner, form clusters of nodes which jointly process gathered information and control the position of nodes for improved sampling of the field.
There is a constant desire to improve the performance of radio systems. Hence there is a need for an improved method of sensing in a radio system employing opportunistic spectrum access. The objective is hence to enhance the detection probability, and/or reduce the false-alarm probability for, for example other users, in a network.